The activated sludge process is a widely-practiced biochemical wastewater treatment and oxidation process that employs microorganisms and oxygen to immobilize dissolved organic pollutant substances in the wastewater as activated sludge, which is partly decomposed into water (H2O) and carbon dioxide (CO2) for removal.
Several challenges are associated with the traditional activated-sludge-based wastewater treatments. For instance, the biochemical cleansing of organic pollutant substances depends largely on the quantity of microorganisms (return sludge), the density of the microorganisms, and the degree of their activity. However, to increase the quantity of microorganisms, their density, and their activity, increasing accordingly the supply of dissolved oxygen, which is essential to the microorganisms, is necessary. Without adequate supply of dissolved oxygen, the wastewater treatment may not be effective.
The importance of the dissolved oxygen can be seen from the following example. In the activated sludge method, 1BOD (Biological Oxygen Demand quantity, mg/l) of organic pollutant is defined as the amount of organic pollutant which requires 1DO (mg/l) of dissolved oxygen (O2) to be broken down by microorganisms in a five-day period under normal atmospheric pressure at 20° C. Meanwhile, 1COD (Chemical Oxygen Demand quantity, mg/l) is defined similarly as the amount which requires 1DO (mg/l) of dissolved oxygen (O2) to be broken down chemically in a 30-minute-to-two-hour period under normal atmospheric pressure at 20° C. Accordingly, in wastewater treatment under the standard activated sludge method, the cleansing capacity achieved per 1DO (dissolved oxygen, mg/l) is no greater than “1BOD (Biological Oxygen Demand quantity, mg/l) of pollutant”. Meanwhile, in the same way, “1COD (Chemical Oxygen Demand quantity, mg/l) of pollutant” also requires 1DO (mg/l) of dissolved oxygen (O2). In other words, to clean “1BOD of pollutant” and “1COD of pollutant” requires 2DO (mg/l) of dissolved oxygen. Without the sufficient amount of oxygen, the effectiveness of the activated sludge wastewater treatment is significantly limited.
Further, the cleansing of wastewater depends fundamentally on the activity of microorganisms (activated sludge), and is thus saddled with the problem of the formation of excess sludge due to the excessive reproduction of these microorganisms, and technology to control this excess has not yet adequately been realized. In other words, the microorganisms which are involved in the cleansing of wastewater are constantly reproducing themselves and then perishing due to self-oxidization, hence controlling and managing the amount of sludge produced and the amount destroyed is extremely difficult, and the lack of this control and management is considered the critical problem of the activated sludge method. As a result, the large quantities of excess sludge that form are concentrated, transported and incinerated or buried in landfills, causing massive processing costs for the removal of excess sludge and emissions problems from the release of carbon dioxide during incineration.
In addition, the control and management of the wastewater cleansing process in the activated sludge method involves numerous parameters which must be observed, with many observation items and observation frequencies, and requires the daily accumulation of a huge amount of data. Furthermore, controlling and managing the treatment capacity of the microorganisms which form the basis of the cleansing process is difficult, and even with the introduction of information technology, analyzing ever-more complicated data, deciding on countermeasures and instructing staff present a heavy burden for wastewater treatment plant managers.
Existing technologies fail to adequately address these challenges. For example, a technique known as preliminary aeration exists to enhance wastewater treatment capacity, in which the return sludge is aerated in advance, the sludge (microorganisms) is activated, and is pumped into the aeration vessel. However, the incremental capacity achieved by preliminary aeration is less than 30%. Because preliminary aeration has such low aeration efficiency, the cost is very high, such that preliminary aeration requires an additional 100% of the aeration cost. Due to the high cost to achieve an incremental capacity of only 30%, preliminary aeration is not cost-effective.
Similarly, another technique used today is long-term continuous aeration bubbling technology which uses bubbles of around 1 mm diameter, the quantity of dissolved oxygen merely reaches an unsaturated state of DO value 2 (mg/l), which is insufficient to bring about a large increase in microorganisms (return sludge).
Likewise, U.S. Pat. No. 7,105,092, issued Sep. 12, 2006, to Kousuke Chiba (“'092 patent”), the disclosure of which is incorporated by reference, discloses a sewage treatment process by which activated-sludge method comprising line atomizing treatment. Wastewater is introduced into the treatment line. The wastewater passes through the adjustment vessel and the sedimentation vessel where inorganic pollutant substances are removed. Subsequently, the wastewater enters the anaerobic reaction vessel where the wastewater is acted upon by anaerobic microorganisms. Subsequently, the wastewater enters the aerobic reactive vessel where organic matter within the wastewater is converted into activated sludge by the action of aerobic microorganisms. After the conversion process in the aerobic reaction vessel, the treated wastewater solution which has had the dissolved organic matter converted into activated sludge is sent together with the activated sludge to the sludge sedimentation vessel, and the supernatant water is expelled from the wastewater treatment system. The supernatant water may also be subjected to advanced treatment for further purification.
The '092 patent further discloses that a portion of the activated sludge which has settled in the sludge sedimentation vessel passes through the sludge intake pipe and is supplied respectively as return sludge to the adjustment vessel, sedimentation vessel, anaerobic reactive vessel, aerobic reactive vessel, and sludge sedimentation vessel to effect multiple functionality for each of those vessels, and to enhance the treatment capacity of the wastewater system while allowing the remainder of the activated sludge to undergo separate treatment as excess sludge. However, each vessel has an original function and role, and in many cases, adding activated return sludge which holds large quantities of reactive gases (oxygen or oxygen with trace amounts of ozone) may interfere with those functions or roles, thus decreasing the effectiveness of wastewater treatment.
Accordingly, there is a need for a way to increase control over activated sludge-based wastewater purification, including optimizing amount of oxygen available for the biochemical reaction during the purification.